State the three components of a mononucleotide.
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pentose sugar; phosphate group; nitrogenous base
Dna Rna And Protein Synthesis
Describe one structural difference between deoxyribose and ribose.
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ribose has a hydroxyl (-OH) group on carbon 2 / deoxyribose lacks an -OH on carbon 2
Identify which bases are purines and which are pyrimidines.
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purines: adenine and guanine; pyrimidines: cytosine, thymine and uracil
Name the bond that links mononucleotides in a polynucleotide chain.
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phosphodiester bond
Describe how a phosphodiester bond forms between two mononucleotides.
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condensation reaction; between phosphate group of one nucleotide and sugar (carbon 3) of next nucleotide; water molecule is released
Compare the structure of DNA and RNA.
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DNA is double-stranded / RNA is single-stranded; DNA contains deoxyribose / RNA contains ribose; DNA contains thymine / RNA contains uracil
State the type of bond between complementary base pairs in DNA.
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hydrogen bond
Describe how complementary base pairing maintains a constant width along the DNA double helix.
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a purine (A or G) always pairs with a pyrimidine (T or C); one large + one small base keeps the total width consistent
Explain why DNA molecules are stable despite being held together by weak hydrogen bonds.
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each hydrogen bond is weak; but very large numbers of bonds along the molecule provide great cumulative strength / total stability
Define semi-conservative replication.
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each new DNA molecule contains one original (parental) strand; and one newly synthesised (daughter) strand
Describe the role of DNA polymerase in DNA replication.
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catalyses the formation of phosphodiester bonds; between adjacent (free) nucleotides on the template strand
Explain how complementary base pairing ensures DNA replication is accurate.
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A only pairs with T / C only pairs with G; free nucleotides align with bases on the template strand; therefore the new strand is an exact complementary copy of the template
Explain why Meselson and Stahl used two different isotopes of nitrogen.
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¹⁵N (heavy) and ¹⁴N (light) have different masses; allowing DNA from different generations to be distinguished by density when centrifuged
After one round of replication in ¹⁴N medium, all DNA appeared as a single intermediate band. Explain how this result supported semi-conservative replication.
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each new DNA molecule contained one original ¹⁵N strand and one new ¹⁴N strand; making each molecule of intermediate density; conservative model would have given two bands (heavy and light), so was ruled out
Discuss how Meselson and Stahl’s experiment supported semi-conservative DNA replication and refuted the conservative and dispersive models. Use your knowledge of the predicted and observed band patterns to support your answer.
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conservative model predicted two bands (heavy + light) after first replication, but only one intermediate band was observed, refuting conservative; semi-conservative predicted one intermediate band after first replication, matching the observed result; semi-conservative also predicted two bands (intermediate + light) after second replication, again matching observations; dispersive model predicted a gradually lightening single intermediate band, but two distinct bands appeared, refuting dispersive; only the semi-conservative prediction matched all observations; experiment provided strong evidence by directly testing all three competing models against measurable density bands
Explain what is meant by a triplet code.
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each amino acid is coded for by a sequence of three (consecutive) DNA bases / a triplet of bases
Calculate the number of different triplets that can be made from four bases.
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A triplet is an ordered sequence of 3 positions, and each position can be filled by any one of the 4 bases (A, T/U, C, G), with repetition allowed. Since the choices at each position are independent, multiply the number of options per position:
Number of triplets = 4 × 4 × 4 = 4^3 = 64
Final answer: 64 different triplets.
Explain why the genetic code is described as degenerate.
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most amino acids are coded for by more than one triplet; reduces the effect of (some) point mutations
Explain the role of start and stop codons in protein synthesis.
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start codon (AUG) signals where translation begins / where the ribosome starts reading; stop codon signals the end of the polypeptide chain / where translation stops
Define the term gene.
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a sequence of DNA bases / sequence of nucleotides; that codes for a (sequence of amino acids in a) polypeptide